1. Field of the Invention
The present invention relates to a cellulose acylate composition, a cellulose acylate film, and their applications.
2. Related Art
Heretofore, cellulose acylate films have been widely utilized as polarizer protective films for liquid-crystal display devices, as having transparency and toughness. For example, proposed are optical films of fatty acid acyl cellulose esters such as cellulose acetate propionate, cellulose acetate butyrate, etc. (see JPA No. 2000-352620). Also proposed are optical films of aromatic acyl-substituted cellulose such as cellulose acetate benzoate (see JPA No. 2006-328298).
However, the optical performance of these films is limited, and for example, films having a negative thickness-direction retardation (Rth) of a large absolute value could not be obtained. Accordingly, these are not satisfactory as retardation films fully applicable to various types of liquid-crystal systems.
For example, in an in-plane switching (IPS) mode where an electric field parallel to the substrates is applied to the liquid crystal, it is proposed to dispose an optical compensatory material having a specific birefringent characteristics, which is prepared by combining a film having a positive birefringence and having an optical axis in the plane of the film and a film having a positive birefringence and having an optical axis in the normal direction of the film, as an optical compensation film, between the liquid-crystal layer and the polarizer, as one method of improving the color tone and the viewing angle aptitude in the black state (see JPA No. hei 11-133408). This optical compensatory method requires a retardation film having a negative thickness-direction retardation of a large absolute value.
In these methods, however, Rth of the film could not be fully reduced; and a method capable of further increasing the absolute value of negative Rth of film is desired. In addition, the cellulose acylate film produced according to the above method has other problems in that its water permeability is large, its water content is high and the humidity dependence of the retardation of the film is great.